Electroless plated nickel on zirconia

ABSTRACT

The invention is an electroformed coating of nickel formed on yttria-stabilized tetragonal polycrystalline ceramic where a hydrofluoric acid etch is utilized with the component during processing to result in an adherent, dense nickel-rich coating. Control of the sensitizing, catalyzing, and reaction enhancement processes to about 90° C. provides improved nickel deposition properties.

This application claims the benefit under Title 35 USC 119(e) of U.S.Provisional application 60/522,941 filed Nov. 23, 2004.

BACKGROUND OF THE INVENTION

This invention relates to a material and method of plating nickel onzirconia ceramic.

DESCRIPTION OF RELATED ART INCLUDING INFORMATION DISCLOSED UNDER 37 CFR1.97 AND 1.98

Presented in FIG. 1 is the known method of forming a component assembly2 by placing an interlayer material 8, preferably nickel metal foil,between a ceramic part 6 and a metal part 4 where a force 10 is appliedto compress the metal foil while the assembly is heated to bond bybrazing. See Fey, et al., U.S. Pat. 6,521,350, which is incorporatedherein by reference in its entirety.

It is well known to electrolytically plate nickel on various electricalconductors. Electroless plating of nickel is also well known, however,applying a thin and adherent coating of nickel on zirconia ceramic isnot known. Obtaining a thin, adherent nickel coating on zirconia hasbeen difficult to achieve using other known approaches. Such a coatingwould offer advantages in the assembly of intricate parts assembliesthat will be bonded by brazing, where the presence of a nickel foil hasbeen used to advantage.

Once one recognizes that an adherent nickel coating of about 10 to 16microns (i.e., 0.0004 to 0.0006 inches) thickness is desired to beplaced on a zirconia ceramic part, then plating becomes a preferredapproach. Electroless nickel is a nickel-phosphorus alloy, making directcomparisons with electrolytic nickel difficult. Differences between anelectroless nickel deposit and electrolytic nickel deposit are due tothe difference in composition of the deposit and to inherent differencesbetween chemical and electrolytic reduction.

A principal advantage of electroless nickel solutions over electrolyticnickel solutions is the lower loss due to the low concentration ofnickel in electroless solutions. Absence of anodes in electroless nickelplating eliminates a source of electrolyte contamination which ispresent in electrolytic plating.

Solution volume is a concern in electrolytic plating. A high ratio ofsurface area of work to volume of bath is required to maximize speed andefficiency of deposition. It is sometimes advantageous to design thetank with a shape suitable to the work.

One useful feature of electroless nickel plating is the high degree ofuniformity in thickness of deposit, as contrasted with electrolyticnickel plating. On a properly catalyzed surface, the driving potentialfor chemical reduction is essentially constant at all points on thesurface. Alternately, in electrolytic plating the amount of plating isdetermined by the local current density, which often varies considerablyfrom point to point on the surface of the work. It is necessary toprovide good agitation to insure uniform nickel ion distribution in thearea adjacent to the work in order to obtain uniform coating thicknesswhen depositing thick electroless nickel coatings. Electroless nickeloffers advantages when plating irregularly shaped objects, such asholes, recesses, or the inside of tubes. Not only are uniform coatingthicknesses obtained, but the need for conforming anodes, thieves,shields, and bipolar electrodes is eliminated. Sharp corners and edgeswill not build up as happens in electrolytic nickel plating.

Since electroless nickel plating is catalytically controlled, thedeposition is localized as desired. Compared to electrolytic nickelplating, the need for a catalyst is both advantageous anddisadvantageous. If the part to be plated is a catalytic metal, it isreadily plated with electroless nickel; if noncatalytic, the reactionmay be initiated by first seeding the surface with a catalytic metal,such as nickel. This seeding may be accomplished electrolytically or bychemical displacement. If the noncatalytic metal is more noble thannickel, a simple method for initiating plating is to immerse the part inthe bath in contact with a more active metal, such as aluminum. If thesurface is nonmetallic, the reaction is catalyzed with a salt, such aspalladium chloride. While certain nonconductors such as some ceramics,glasses, plastics, or wood are catalyzed and plated with electrolessnickel, the successful plating of nickel on zirconia is unreported.Nickel electroplating requires that the surface first be renderedconductive, something which is difficult to accomplish on intricatelyshaped parts or on zirconia ceramic.

The differences in physical properties of electroless versuselectrolytic nickel deposits reflect the fact that electroless nickeldeposits contain 3% to 15% phosphorus. Electrodepositednickel-phosphorus alloys possess many physical properties closelyresembling those of electroless nickel deposits. Compared toelectrolytic nickel deposits, electroless nickel deposits are amorphousin structure, harder (500 Vickers in the as-plated condition compared to900 Vickers after heat treatment), and more brittle. Electroless nickeldeposits are usually smooth, semi-bright to bright in appearance, andpossess a laminar structure similar to bright nickel. Electroless nickeldeposits do not possess the full luster of bright nickelelectrodeposits.

The corrosion resistance of a given thickness of electroless nickel isusually superior to an equal electrolytic deposit for the followingreasons: the greater uniformity in thickness of electroless deposits,eliminating the need of over plating to provide adequate corrosionprotection for recessed areas; the virtual absence of porosity inelectroless deposits; the homogeneous structure (that is, no crystalboundaries) of electroless deposits; and the greater corrosionresistance of nickel phosphite.

A need exists for an adherent, thin electroless nickel coating onzirconia, which will facilitate bonding by, for example, brazing toother metals.

BRIEF SUMMARY OF THE INVENTION

It has now been discovered that a method for coating a ceramiccomponent, consists of the steps of selecting the component made ofyttria-stabilized tetragonal zirconia, etching the yttria-stabilizedtetragonal zirconia polycrystal ceramic component in hydrofluoric acid,rinsing the ceramic component in methanol, sensitizing said ceramiccomponent, catalyzing the ceramic component, treating the ceramiccomponent to reaction enhance the ceramic component, placing the ceramiccomponent in a plating bath that is configured for forming anelectroformed nickel coating thereby forming a nickel coating on saidceramic component; and measuring the thickness of the nickel coating.

The method may also include etching the yttria-stabilized tetragonalzirconia polycrystal ceramic component in 40% concentrated hydrofluoricacid for 30 seconds.

Control of the sensitizing, catalyzing, and reaction enhancement bathsto about 90° C. has been demonstrated to provide improved nickeldeposition.

The novel features of the invention are set forth with particularity inthe appended claims. The invention will be best understood from thefollowing description when read in conjunction with the accompanyingdrawings.

OBJECTS OF THE INVENTION

It is an object of the invention to apply a thin, adherent coating ofnickel on zirconia ceramic by electroforming.

Other objects, advantages and novel features of the present inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a component assembly formed by the known nickel foil brazingapproach.

FIG. 2 is a component assembly formed by an electroformed nickel foilcoating.

FIG. 3 illustrates a flow chart of the electroless plating process.

DETAILED DESCRIPTION OF THE INVENTION

The invention is presented in FIG. 2 in one preferred application wherea component assembly 28 is suitable for bonding by brazing. A ceramicpart 6 has an intimately bonded electroformed nickel coating 22 on oneend that is placed in intimate contact with metal part 4, which ispreferably titanium or a titanium metal alloy. The ceramic part 6 ispreferably yttria-stabilized tetragonal zirconia polycrystal (Y-TZP).The assembly 28 is brazed while the electroformed nickel coating 22 isheld in intimate contact with the metal part 4 by force 20. Such anassembly is suitable for implantation in living tissue where thecomponent assembly 28 must be biocompatible. Such implantable devicesare well known and in a preferred embodiment they are very small andimplantable by injection, preferably having an outer diameter of 6 mm orless and an overall length of 60 mm or less. See U.S. Pat. Nos.4,991,582, 5,193,539, 5,193,540, 5,324,316, 6,185,452, 6,208,894, and6,315,721, for example. The instant invention has broad applicabilityand is not limited to such applications, however.

A first step in the process of the invention, as presented in FIG. 3 isto select a Y-TZP ceramic component 102 to which an electroformed nickellayer will be bonded by direct deposition on a selected surface of theceramic component. The deposition process is not limited toline-of-sight deposition and the coating thickness on complex shapedparts can be controlled during the electroforming process by the use ofbaffles, for example. Y-TZP materials are widely known and are a phasestabilized form of yttria ceramic that has a low thermal expansioncoefficient and excellent stability in certain harsh environments, suchas when implanted in living tissue.

A second step is to clean the Y-TZP component using a detergent cleaner104, such as Alconox®, available from Alconox, Inc. of White Plains,N.Y. The component is rinsed with deionized water and second rinsed withisopropyl alcohol using ultrasonic cleaning.

The component is then masked in step 106 leaving the area of thecomponent that is to be coated with electroformed nickel unmasked. For ahollow shaped component, such as a hollow tube or open cylinder, theopening may be plugged with silicone. The component surface to beprotected is preferably covered with Kynar® PVDF shrink tubing,available from 3M.

The inventors have found that to achieve success, the next step in apreferred embodiment is to etch the Y-TZP ceramic in hydrofluoric acid,step 108, at a concentration of 40% hydrofluoric acid for 30 seconds. Aknown source of hydrofluoric acid is available from Alfa Aesar, WardHill, Mass. The component is then rinsed in deionized water.

The component is then placed in a sensitizer treatment, step 110, ofstannous chloride (SnCl₂) at a concentration of 7% (i.e., 70 g ofanhydrous SnCl₂ combined with 40 ml of 36.5% concentration hydrochloricacid plus deionized water to make one liter of solution) for about 7minutes. In a preferred embodiment, the solution is held at 90° C.±2° C.This solution is unstable and cannot be stored more than one day. Thecomponent is then rinsed in deionized water.

The component is next placed in a catalyzer treatment, step 112, ofPdCl₂ for about 4 minutes. The solution is about 1 gram/liter of PdCl₂plus 20 ml of 36.5% concentrated hydrochloric acid and 40 ml of 40%concentration hydrofluoric acid combined with deionized water to make 1liter of solution. In a preferred embodiment, the solution is held at90° C.±2° C. This solution has been found to be stable. The component isthen rinsed in deionized water.

Next the component is placed in a reaction enhancement treatment, step114, (also known as a reducing agent) at 90° C.±20 for 10 seconds. Thesolution is 20% sodium hypophosphite, NaH₂PO₂. This is 200 grams perliter of deionized water where the water is added to make one liter ofsolution.

The prepared component is next placed in a plating bath, step 118, at88° C.±2° C. or preferably at 88° C.±1° C. The inventors prefer theAdvanced High Phosphorus, Semi-Bright Electroless Nickel System, Stock #44305 from Alfa Aesar. The electroforming process proceeds according toparameters and procedures that are known to those skilled in the art.The deposition rate is preferably about 0.0004 to 0.0006 inches perhour.

The inventors have found, step 120, that, in their particularconfiguration, it is preferable to place the component in a flow ofbubbling air that is about 0.4 to 0.5 cm between the nearest portion ofthe component and the source of bubbles. The flow rate is characterizedas slow and is controlled to remove the hydrogen generated from thereaction to facilitate deposition of nickel.

In step 122 the part is removed from the bath, optionally rinsed indeionized water before being rinsed with methanol. The part is rinsed indeionized water before a sensitizer treatment is conducted in step 124for 15 to 30 seconds, rinsed in deionized water, followed by a catalyzertreatment for 15 to 30 seconds in step 126 followed by a deionized waterrinse. The component is subject to the reaction enhancement treatment instep 128 for 10 seconds at 90° C.±2° C.

The component is returned to the plating bath in step 130 for 10 minutesin bubbling air, step 132, after which it is optionally subjected tosteps 122 to 132 again, the first time through the process, if it isdesired to increase the deposition thickness of the nickel. Thecomponent is deionized water rinsed in step 142, methanol rinsed in step144, rinsed in deionized water and again rinsed for about 15 seconds in40% hydrofluoric acid in step 146 before being rinsed in deionized waterin step 148 and returned to the plating bath in step 150, after which itis optionally returned the first time through the process, if it isdesired to increase the deposition thickness of the nickel, to repeatsteps 142 to 150 prior to being removed from the bath, rinsed indeionized water, and having its thickness measured by known means, step152, and the adhesion measured by known means, step 154, to assure thatthe nickel to ceramic bond is acceptable.

Thus, in accordance with this invention, it is now possible to apply acoating of nickel by electroforming directly on a Y-TZP ceramiccomponent. This is surprising since prior investigators have not beenable to successfully accomplish this coating process.

The following example is submitted to illustrate but not to limit thisinvention. Unless otherwise indicated, all parts and percentages in thespecification and claims are based upon weight.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. It is therefore to beunderstood that, within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described.

1. A method for coating a ceramic component, comprising the steps of:selecting said component comprised of yttria-stabilized tetragonalzirconia; etching said yttria-stabilized tetragonal zirconia polycrystalceramic component in hydrofluoric acid; rinsing said ceramic componentin methanol; sensitizing said ceramic component; catalyzing said ceramiccomponent; reaction enhancement treating said ceramic component; placingsaid ceramic component in a plating bath that is configured for formingan electroformed nickel coating thereby forming a nickel coating on saidceramic component; and measuring the thickness of said nickel coating.2. The method according to claim 1, wherein etching saidyttria-stabilized tetragonal zirconia polycrystal ceramic component inhydrofluoric acid is etching in hydrofluoric acid for about 30 seconds.3. The method according to claim 2, wherein said hydrofluoric acid isabout 40% concentrated hydrofluoric acid.
 4. The nickel coating formedby the method of claim
 1. 5. The method according to claim 1, whereinsaid sensitizing is performed in a solution comprising about 70 grams onSnCl₂ and 40 ml of concentrated hydrochloric acid in a one litersolution.
 6. The method according to claim 1, wherein said catalyzing isperformed in a solution comprising about 1 gram of PdCl₂, 20 ml ofhydrochloric acid, 40 ml of hydrofluoric acid in a one liter solution.7. The method according to claim 1, wherein said reaction enhancementtreating is at about 90° C.
 8. The method according to claim 1, whereinsaid placing said ceramic component in a plating bath is at about 88° C.9. The method according to claim 1, wherein said sensitizing saidceramic component is at about 90° C.
 10. The method according to claim1, wherein said catalyzing said ceramic component is at about 90° C. 11.The method according to claim 1, wherein said reaction enhancementtreating said ceramic component is at about 90° C.